1. Field of the Invention
The present invention relates generally to an improved heat conditioning technology for thermoplastic resin products, and more particularly to an improved heat conditioning or fixing treatment for use with the thermoplastic resin products, which is applicable in practice to pneumatic pressure forming, vacuum-pneumatic pressure forming or like thermoforming of general plastic film and sheet products, etc.
2. Description of the Prior Art
There is generally known a variety of containers which are thermoformed from thermoplastic film and/or sheet products and which are extensively used as containers for containing foodstuff products. Also, there is known a process by which such containers are formed thermally from thermoplastic film or sheet materials by way of the vacuum forming, pneumatic pressure forming, or by way of combination of these two forming processes.
Here, before going any further, it will be helpful for the readers to give in summary a general introductory explanation on certain specific matters concerned with the prior art vacuum-and-pneumatic pressure processes and apparatus for thermally forming the thermoplastic resin products, for a better understanding of the present invention.
FIG. 1 is a general schematic view showing the general outline of the conventional vacuum and pneumatic-pressure thermoforming machines and of the conventional processes of thermoforming. FIGS. 2, 3 and 4 are schematic perspective views showing the general state of a series of containers formed in a matrix relationship in a sheet of conventional thermoplastic material, showing a plastic container product stamped away from the sheet of thermoplastic material including the series of containers therein, and showing a scrap of the thermoplastic sheet from which the plastic containers have been stamped away, respectively.
Now, in the drawing figures mentioned above, there is shown a roll of plastic film or sheet material designated at the reference numeral 1, which is in an elongated web form and is wound in a roll fashion to be supplied for the series of thermoforming processes to follow. There is unrolled a sheet of plastic sheet material designated at 2 out of the roll of plastic material 1. There are also shown a pair of heating panels designated at 3 and 4, which are disposed operatively in an opposed relationship with respect to each other and which are designed to heat the sheet of plastic material 2 to its softening point through a temperature range characteristic of the plastic so as to be formable or moldable thermally while passing therebetween. Also shown is a forming mold at the reference numeral 5 having passageways for sucking or pumping out air at 14 while generating a vacuum state in the interior of the forming mold 5. There are also provided cooling and heating means not shown.
There are disposed a series of plugs designated at the reference numeral 6 in an opposed relationship respectively with the series of molding cavities of the forming mold 5, which are adapted to force the web of plastic sheet 2 into each of the molding cavities of the forming mold 5 for a preliminary molding procedure and which are provided with a plurality of air blowing passageways 12. For containers with a shallow depth, it is sometimes the case that the containers may be formed to their designed shapes without using these plugs. There is shown a container at 7 in FIGS. 1, 2 and 3, which is a typical representation of a container product formed in a thermoplastic manner.
A scrap of plastic sheet is shown at the reference numeral 8 in FIG. 4, from which a plurality of containers 7 have been stamped away and in which there are left a plurality of openings after the stamping operation. Also shown in FIG. 1 are upper and lower stamping dies designated at 9, 10 disposed in an opposedly paired relationship with respect to each other, a pneumatic control device at 11 for the generation and control of compressed air and for the shifting control of air suction operation, a vacuum generating device at 13, a plastic sheet carrier 15 for carrying the sheet of plastic material in the interior of a vacuum-and-pneumatic pressure forming device, a vacuum-and-pneumatic pressure forming device 18, and a plastic product stamping unit 19.
Referring more specifically to the thermoplastic forming process by way of FIG. 1, firstly there is seen unrolled intermittently a sheet of plastic material 2 out of the roll of plastic material 1 by means of the plastic sheet carrier 15. Subsequently, as it moves between heat panels 3 and 4, the sheet of plastic material 2 is heated to its softening point or its molding temperature by the paired upper and lower heating panels 3, 4 so as to be formed thermoplastically. There is sometimes a case that the sheet of plastic material 2 is heated on one side only with either of the upper and lower heating panels 3, 4, where so desired. At the following step, the forming mold 5 and the plurality of plugs 6 are then caused to move toward each other or in the direction shown by the arrows X1 and Y1, respectively, so that the sheet of plastic material 2 may be sandwiched and clamped therebetween to the designed molding shape of a formed plastic product, accordingly.
In the next stage, the steps of pumping out the air in the openings or cavities of the forming mold 5 through the passageways 14 and of air blowing forced in compression into the forming mold cavities are then taken concurrently so as to force the sheet of plastic material 2 into the cavities of the forming mold 5 and have the plastic sheet formed to the desired shape of a container 7, accordingly.
Next, thus-formed containers 7 are then cooled off by cooling the forming mold 5 while keeping the containers in close contact position against the interior of the molding cavities by the aid of air blowing in compression from the companion plugs 6.
At the following step, the forming mold 5 and the companion plugs 6 are caused to move backwardly away from their engaging positions in the directions as shown by the arrows X2 and Y2, respectively, so as to have the forming mold 5 opened, and the containers 7 formed in a matrix fashion in the sheet of plastic material 2 are now ready to be removed out of the forming mold 5 together with their matrix formation as it is. (see FIG. 2.)
After the containers 7 are removed together out of the forming mold 5, they are carried to the formed product stamping unit 19, where they are separated or stamped to their individual or piece shape from the sheet of plastic material 2 by operation of the pair of upper and lower stamping dies 9, 10. More specifically, the sheet of plastic material 2 with the matrix of containers formed therein is now stamped to be separated into the containers 7 shown in FIG. 3 and the plastic sheet scrap 8 shown in FIG. 4, respectively.
Such containers as obtained from the vacuum and pneumatic forming processes of the conventional art as noted above in connection with FIG. 1, may hold their shapes in practice as long as they are used in a normal temperature range. However, there is left an inevitable drawback in such containers according to the conventional forming processes as noted above; that is, when these containers are subjected to a certain high temperature while being charged with a hot content, or sterilized in a high temperature environment after charged with a content, the containers would be deformed owing to its thermoremanent strain from the thermoforming procedures, which would not only spoil the goods' value, but would no longer serve any due function of a container.
In order to overcome such drawback, which is particular to the conventional thermoforming processes, it is then essential to provide these products with a proper thermal resistance. In order to provide the required heat-resisting properties, it is the generally known practice for fibrous and film products that they heated in the presence of tension for the purpose of thermally fixing them.
The present invention is essentially directed to a resolution to such inconveniences and difficulties as outlined above and experienced in the adoption of the conventional thermoforming processes.